1. Field of the Invention
An internal combustion engine has been developed with improved charging efficiency obtained by a supercharger, such as a mechanical supercharger, driven from the engine crankshaft through belts, gears or chains, and a turbocharger, driven by exhaust gases of the engine. There are two types of mechanical superchargers. These types are a compressor type of supercharger which compresses intake air and a volume or blower type of supercharger, which does not itself compress intake air.
A compressor type supercharger (which is referred to simply as a "compressor supercharger") provides a high supercharge pressure when the engine operates at higher, or heavier loads. In this manner, the supercharger improves charging efficiency or raises engine power as compared to a volume type supercharger. Further, the compressor supercharger, when the engine operates at lower, or lighter, loads and moderate loads, raises the temperature of intake air, so as to cause a decrease in pumping loss of the engine and to improve fuel vaporization or fuel atomization. This results in an improvement in fuel economy and a decrease in the hydrocarbon (HC) content of emission gases.
On the other hand, the compressor supercharger has the disadvantage of requiring a higher driving power higher than a volume type supercharger, since it is necessary to compress intake air. Due to this fact, the compressor supercharger increases driving loss while the engine operates at lower, or lighter, loads, and provides less engine power.
2. Description of Related Art
To reduce the driving loss of the volume type supercharger, it is typical, as is known from, for instance, Japanese Unexamined Utility Model Publication No. 63-51121, to provide an intake system having a bypass passage with a control valve bypassing the supercharger. The control valve is actuated to open the bypass passage at lower, or lighter, engine loads so as to decrease driving loss of the supercharger.
The fuel economy and emission gas properties of the engine can be improved, in particular, at lower engine loads by using a compressor supercharger, which provides the advantage of raising the temperature of intake air. However, during the operation of the engine at higher, or heavier, engine loads, wherein the intake air is compressed to a high pressure, the temperature of the compressed intake air is raised to excess, so as to often cause undesirable or abnormal fuel combustion. For this reason, it is essential to install an inter-cooler in the intake passage downstream of the compressor supercharger and to control the cooling temperature of intake air according to operating engine loads.
Compared with the intake system with a volume type supercharger, in which, when the engine operates at lower, or lighter, loads, intake air flows so as to bypass the supercharger loads and effectively decrease the driving loss of the supercharger, a system having a compressor supercharger tends to waste driving power. This is because of the fact that a compressor supercharger, such as, in particular, a compressor supercharger always driven by the engine, works to compress intake air even at lower engine loads, where the engine has no supercharging demand. Thus, the compressor supercharger uselessly wastes driving power.